This invention relates to a load-bearing non-expansible non-pneumatic tire ("NPT" for brevity) having an improved "ride" which is due to an annular elastomeric insert, referred to as a "ride-enhancing insert", "ride insert" or "insert" for brevity, because it is captured within the web, either directly beneath the circumferential center line of the web, or as a laminate within the web in which the outer periphery of the insert extends above the circumferential center line of the web. In either case, the insert circumferentially embraces the outer surface of a wheel rim on which the NPT is mounted. The acronym "NPT" is used herein to denote any non-pneumatic tire which comprises a central load-bearing web of elastomeric material.
The structure of the NPT itself is not narrowly critical so long as it includes a load-bearing integral web which has an insert-containing base in common with that of the NPT. Several prior art NPTs have been constructed with load-bearing webs, but NPTs which benefit most from being modified in accordance with this invention are those more fully described in U.S. Pat. No. 4,832,098 to Palinkas and Page, titled "Non-pneumatic Tire with Supporting and Cushioning Means"; U.S. Pat. No. 4,921,029 titled "Trapezoidal Non-Pneumatic Tire with Supporting and Cushioning Means" to Palinkas and Pajtas; U.S. Pat. No. 4,934,425 to Gajewski et al titled "Non-Pneumatic Tire"; U.S. Pat. No. 5,023,040 to Gajewski et al titled "Method for Making a Polyurethane Non-Pneumatic Tire"; and, Japanese patent appln. Heisei 2-179503 to Hirayama, the disclosures of each of which is incorporated by reference thereto as if fully set forth herein.
In each case, the NPT comprises planar, oppositely directed angulated ribs which intersect at the web, on opposite side faces thereof, and the web maintains inner and outer hoops in radially spaced apart relationship. The NPT is made from an elastomeric material, most preferably, polyurethane. In accordance with this invention, each such NPT is formed integrally with the insert.
The novel insert is referred to as a "central ride insert" or "RI" when the web is centrally located on the surface of the wheel rim in the circumferential center plane of a NPT, as in the '098 and '029 patents.
In any wheeled vehicle, particularly an automobile or truck, an NPT with integrally formed ride insert, referred to by the acronym "NPT/RI" for brevity, serves to enhance the comfort of a "ride" afforded by the NPT without the novel RI. As might be expected, the NPT/RI may be mounted on the rim of any conventional wheel rim or other cylindrical mounting surface, to provide a cushioning effect as the wheel travels over a surface.
As is well known, the comfort of a ride in a wheeled vehicle equipped with a prior art NPT having a solid central web, falls notably short of the comfort afforded by the same vehicle equipped with a state-of-the-art radial pneumatic tire. Though noticeable enough at running speeds on a smooth highway, but deemed a minor drawback, the ride becomes uncomfortable when the running wheel encounters a break in the smooth pavement, or the vehicle is operated on a rough road.
The reason is that on a smooth highway, the distribution of the load is so uniform that the angulated ribs of the NPT absorb and distribute minor shocks first transmitted through the outer hoop. When the wheel encounters an obstacle which generates a major shock, enough of the load is transmitted directly through the web to create a feeling of considerable discomfort. The reason is that the vertical web buckles, because it is essentially incompressible, despite being prevented from bending because of the ribs on either side of the web. Such buckling serves to improve the ride, but it was found that the ride is greatly improved if the major shocks encountered by the wheel can be absorbed better than they are with a conventional central web. The problem is to improve the comfort of the ride afforded by the NPT without decreasing its structural strength and without denigrating its other highly desirable characteristics.
The obvious choice for solving the problem without interfering with the structural configuration of the NPT is to provide it with a thick soft tread. Except that such a tread would have so short an operating life as to be impractical. Any other solution would necessarily have to make some change to the existing structure of the NPT. To insert an elastomeric member into a NPT is conceptually contraindicated because of the known build-up of heat of the NPT in the central zone during operation. Yet we have found that the presence of the RI does not appear to change the existing function of the web as a load-bearing member during normal operation but allows the web also to function as a shock absorber. Because the RI is positioned at the base of the web it is not adversely affected by the heat build-up in the central portion of the NPT; and, because the RI is centrally disposed on the outer surface of a wheel rim on which the NPT is formed, the zone containing the RI ("RI zone") may be vented.
Referring to the '098 and '029 patents, inter alia, it will be evident that the effectiveness with which the NPT distributed the load, was primarily attributable to its unitary annular ("hoop") construction which transmitted and distributed the static and dynamic load throughout the hoop structure, eventually to the inner hoop which was anchored to the wheel rim. Since, in a typical situation, the wheel rim is on a driven wheel of a vehicle, the bond between the inner hoop of the NPT and the surface of the wheel rim on which the NPT is mounted, must be able to withstand the torque generated during operation of the vehicle. This creates a problem of securing the NPT to a wheel rim for use as a road wheel, so that the NPT will not be thrown off by centrifugal force. The solution to the problem is simplified in a conventional NPT because the entire area of the inner hoop (circumferential base of the NPT) is adhesively secured to the circumference of the wheel rim.
To improve the ride of the NPT by cushioning it on the wheel rim, the obvious course was to provide a soft, thick tread on the circumference of the NPT to replace the conventional relatively thin solid hard rubber tread, and also to provide an elastomeric base cushion between the NPT and the wheel rim, which base cushion was more compliant than the NPT. The soft, thick tread certainly improves the comfort of the ride because of the "pillow effect", the thicker and softer the tread, the more comfortable the ride. However, a soft, thick tread decreases the handling and roadworthiness of the vehicle. In addition, the longevity of the tread is drastically foreshortened. For the foregoing reasons, a thicker, softer tread is a poor choice.
The alternative, namely providing a cushioning base ("base cushion") presented serious problems with respect to (i) bonding the NPT to the material of choice for the base cushion, and (ii) tolerating the high torque loading in the base due to acceleration and deceleration of the vehicle, even if the bonding between NPT and base cushion was excellent. This solution, in effect, required finding an elastomer which would provide the desired "cushioning" and at the same time would allow the new elastomer to be effectively bonded to the NPT. This search was immediately recognized as being too-daunting an endeavor. Moreover it is not known how one can choose a base cushion which will meld with the peculiar function of undercut, planar, oppositely disposed angulated "ribs" or "spokes" in the NPT, which provide a construction to ensure the ribs will not bend. Instead of bending, they buckle when the critical load is exceeded. It is this peculiar characteristic cushioning due to compressive deformation of the NPT in normal use, coupled with buckling of the ribs when the critical load is exceeded, which is instrumental in giving the NPT `ride` and `handling` characteristics which can come close to mimicking those of a conventional pneumatic tire under normal operating conditions on a smooth highway. No laminate of elastomers is known to provide an operable, let alone reliable and durable, NPT.
This invention is directed to a solution which does not vitiate the durability of the NPT, does not diminish its handling characteristics, yet greatly improves its "ride".